The present invention relates to an improvement in a valve of the type which includes a valve body having a cavity formed therein, a valve element rotatably mounted in said cavity and valve seats interposed between said valve element and said valve body in which a fluid passage can be opened and closed by rotating the valve element while maintaining fluid seal by means of said valve seats tightly held against the valve element. More particularly, it relates to an improvement in the construction of such valve seat.
The valve seat is usually made of synthetic resin, synthetic rubber or like material. In such type of valve, metal chips, scales or other foreign materials produced during the piping operation may frequently get into the interface between the valve seats and the valve element to cause damage on the valve seats and/or valve element, resulting in leakage of the fluid through the gap produced therebetween.
In order to prevent such leakage of the fluid, it has been recently proposed to provide an emergency device which includes, in addition to the conventional valve seats or main valve seats, a movable auxiliary valve seat which is normally held in its released position and is actuated into position where the auxiliary valve seat is pressed against the valve element by the action of pressure medium which is supplied when leakage of the main valve seat is produced. Such emergency device does not include a means for returning the auxiliary valve seat into its original position and, therefore, such auxiliary valve seat is tightly pressed against the valve element, after it has been actuated, so that it is impossible or very difficult to rotate the valve element and if the valve element is forcibly rotated under such condition the valve seat may become seriously damaged. Accordingly, such sealing device only serves as an emergency device which temporarily acts to seal the valve when the leakage is produced.
Another type of emergency sealing device has been proposed, which employs a sealant, such as grease, which is fed to the interface between the valve element and the valve seats, when the leakage is produced, to temporarily seal the valve. However such emergency sealing device cannot maintain its sealing action for a long time, that is, the sealant fed to the interface between the valve element and the valve seats becomes easily scattered into the cavity of the valve body under the action of the fluid pressure in the piping and thus cannot maintain its sealing function without supplementing the sealant.
Furthermore an emergency sealing device has been recently proposed, which includes a movable valve seat which is subjected to the fluid pressure to be tightly held against the valve element and an emergency sealant feeding means. This type of the emergency sealing device cannot avoid the disadvantages in the above-mentioned emergency sealing devices.
Accordingly, in the heretofore proposed device it is necessary to repair or substitute the valve seat and/or valve element soon after the emergency device has been actuated.